1. Field of the Invention
The present invention relates to thin film polycrystalline solar cells and methods of forming them.
2. Related Background Art
The thin film polycrystalline Si solar cells have polycrystalline Si films formed by film formation steps such as CVD, epitaxial growth, etc., can be produced at a lower production cost than bulk crystal solar cells produced by forming a semiconductor junction in a wafer, are expected to achieve a higher photoelectric conversion efficiency than a-Si solar cells, and are promising candidates as next generation solar cells. Typical structures of the conventional thin film polycrystalline Si solar cells include those of the pn junction as shown in FIG. 15 or of the pin junction as shown in FIG. 16.
In FIG. 15, numeral 81 designates a substrate also serving as a support, and 82 an electroconductive metal film that also acts as a light reflecting layer. Numeral 83 denotes a polycrystalline Si thin film semiconductor layer doped in a high concentration with an impurity of a conductivity type, which is laid in order to establish good electrical contact between the metal layer 82 and a semiconductor layer 84. Numeral 84 represents a polycrystalline Si thin film semiconductor layer, which is normally doped with a slight amount of an impurity of the same conductivity type as that of the layer 83. Inside this layer 84 a potential distribution is made on the basis of contact with a layer 85, and thus the layer 84 acts as a photocharge generating layer. Numeral 85 indicates a thin film semiconductor layer doped in a high concentration with an impurity of the opposite conductivity type to that of the layers 83 and 84. Numeral 87 denotes an antireflection layer for preventing reflection of light, which is provided for taking in light efficiently. Numeral 86 stands for collecting electrodes for extraction of electric current.
When the solar cells are constructed using films of polycrystalline Si of small crystal grain sizes, the pin structure as shown in FIG. 16 is employed in order to flow the electric current by drift. Numeral 91 designates a substrate also serving as a support, and 92 an electroconductive metal film also acting as a light reflecting layer. Numeral 93 denotes a polycrystalline Si thin film semiconductor layer doped with an impurity of a conductivity type. Numeral 94 represents an intrinsic, polycrystalline Si thin film semiconductor layer.
Numeral 95 represents a thin film semiconductor layer doped with an impurity of the opposite conductivity type to that of the layer 93. An electric field is established in the intrinsic semiconductor layer 94 interposed between the layer 93 and the layer 95, and the charge generated in the layer 94 flows along the electric field. Numeral 97 indicates an antireflection layer for preventing reflection of light, which is provided for taking in light efficiently. Numeral 96 stands for collecting electrodes for extraction of electric current.
The solar cells of such structures can be produced without necessity for slicing and polishing steps, different from the bulk crystal Si solar cells, and thus the production cost can be lower. Since they can also be produced on the substrate of glass, metal, or the like, it is also feasible to perform continuous production. For this reason, they can also be used in the stack structure with the a-Si solar cells and, therefore, the polycrystalline Si thin film semiconductor layers are promising materials as a long-wavelength light absorbing and photocharge generating layer. The reason is that the a-SiGe film also used similarly as a long-wavelength light absorbing and photocharge generating layer has to be made of the high cost source material of GeH4 gas.
The polycrystalline Si solar cells of the structure of FIG. 15 or FIG. 16 were actually produced to evaluate their characteristics and it was shown that short-circuit currents and fill factors were greatly different among samples. None of the samples demonstrated good short-circuit current and fill factor characteristics. To yield solar cells with good characteristics about the both short-circuit current and fill factor was thus a significant subject in the research on the thin film polycrystalline Si solar cells.
An object of the present invention is to provide thin film polycrystalline Si solar cells with good characteristics about both the short-circuit current and fill factor.
According to a first aspect of the present invention, there is provided a thin film polycrystalline solar cell comprising a substrate; a first semiconductor layer provided on the substrate and comprised of Si highly doped with a conductivity-type controlling impurity; a second semiconductor layer provided on the first semiconductor layer and comprised of polycrystalline Si slightly doped with a conductivity-type controlling impurity of the same conductivity type as that of the first semiconductor layer; and a third semiconductor layer provided on the second semiconductor layer and highly doped with a conductivity-type controlling impurity of a conductivity type opposite to that of the impurities for the doping of the first and the second semiconductor layers, wherein crystal grains grown from crystal nuclei generated in the first semiconductor layer are continuously grown to form the first and the second semiconductor layers, are also horizontally grown to contact neighboring crystal grains, and are perpendicularly grown to form an interface with the third semiconductor layer.
According to a second aspect of the present invention, there is provided a thin film polycrystalline solar cell comprising a substrate; a first semiconductor layer provided on the substrate and comprised of Si doped with a conductivity-type controlling impurity; a second semiconductor layer provided on the first semiconductor layer and comprised of Si of an intrinsic conductivity type; and a third semiconductor layer provided on the second semiconductor layer and doped with a conductivity-type controlling impurity of a conductivity type opposite to that of the impurity for the doping of the first semiconductor layer, wherein crystal grains grown from crystal nuclei generated in the first semiconductor layer are continuously grown to form the first and the second semiconductor layers, are also horizontally grown to contact neighboring crystal grains, and are perpendicularly grown to form an interface with the third semiconductor layer.
The first aspect as described above includes a solar cell having the structure of n+/nxe2x88x92/p+ or p+/pxe2x88x92/n+, and the second aspect includes a solar cell having the structure of n/i/p or p/i/n.
A method of forming a thin film polycrystalline solar cell according to the present invention is a method of forming a thin film polycrystalline solar cell by stacking on a substrate a first semiconductor layer of a thin film comprised of Si highly doped with a conductivity-type controlling impurity, stacking thereon a second semiconductor layer of a thin film comprised of polycrystalline Si slightly doped with a conductivity-type controlling impurity of the same conductivity type as that of the first semiconductor layer, and further stacking thereon a third semiconductor layer of a thin film highly doped with a conductivity-type controlling impurity of a conductivity type opposite to that of the impurities for the doping of the first and the second semiconductor layers, thereby forming a solar cell with a semiconductor junction structure of n+/nxe2x88x92/p+ or p+/pxe2x88x92/n+, the method comprising: repeatedly carrying out film deposition and plasma processing to form the first semiconductor layer; then growing crystal grains from crystal nuclei generated in the first semiconductor layer in a direction perpendicular to the substrate and also growing the crystal grains in a horizontal direction until the crystal grains contact neighboring crystal grains, and also effecting continuous growth of the crystal grains in the second semiconductor layer up to an interface with the third semiconductor layer, thereby forming the second semiconductor layer.
In the solar cells and the method described above, preferably, the crystal nucleus density of the crystals perpendicularly grown in the first semiconductor layer is not more than 1xc3x971010 cmxe2x88x923.
In the solar cells and the method described above, preferably, the shape of a region in which each crystal grown from a crystal nucleus is horizontally grown to contact neighboring crystals, in the first semiconductor layer, is a cone (or circular cone) or pyramid having an apex angle of not less than 60xc2x0 in a cross section parallel to the direction of the growth.
The term xe2x80x9cpyramidxe2x80x9d as herein employed is intended to embrace tri- or more angular pyramids including triangular pyramid, quadratic pyramid, pentagonal pyramid, and so on.
As to the description of the location of beginning of crystal growth, the expression xe2x80x9ccrystal growth in a layer (e.g., the first layer)xe2x80x9d as herein employed is intended to mean crystal growth beginning at a location inside the layer and to exclude crystal growth beginning at an interface between the layer and another layer or substrate adjacent to the layer.
In the solar cells and the method described above, preferably, the crystals grown in the second semiconductor layer up to the interface with the third semiconductor layer have an average grain diameter of not less than 100 nm in a direction parallel to the substrate.
In the solar cells and the method described above, preferably, the interface formed between the crystals grown in the second semiconductor layer and the third semiconductor layer has an unevenness of not less than 20 nm.
According to the present invention, the solar cells are those wherein crystal nuclei generate within the first semiconductor layer, the crystal grains are grown in (or as) the second semiconductor layer up to the interface with the third semiconductor layer, and the crystal layer comprised of the crystal grains is used as a generation layer and transport layer of photocharge, and in these solar cells the charge generated in the crystal grains is transported in the direction perpendicular to the substrate without being affected by grain boundaries and a high-resistance layer of nondoped amorphous Si or the like, which can successfully enhance the fill factor characteristic. In addition, the average grain diameter of the crystal grains is not less than 100 nm and the shape of the crystals at the interface is provided with an unevenness of not less than 20 nm, which can successfully attain a large short-circuit current characteristic.